Confused over settings to play 4K videos

That’s a different setting, keep that off. Here are the settings concerned

Settings>System>Display>Resolution> 1920x1080p
Settings>System>Display>Whitelist> (empty) *
Settings>Player>Videos>Adjust display refresh rate> On start/stop

Some televisions may also need, or benefit from, the following being set…

System>Display>Force 4:2:2 colour subsampling> (enable)

With the above settings your UI will be output in Full HD and your 4K content will be output in 4K. *Information regarding the whitelist can be found here. If you have any doubt, feel free to upload some logs so we can verify that your settings are indeed correct.